Recently, as the mobile devices provide higher and higher level of performances and a wide and wider variety of functions, it has been desired that cells used as power supplies of mobile devices have larger capacities. As a secondary cell fulfilling such a requirement, a nonaqueous electrolytic secondary cell is a target of attention. In order to increase the capacitance of the nonaqueous electrolytic secondary cell, it is proposed to use silicon (Si), germanium (Ge), tin (Sn) or the like as an electrolytic active material (hereinafter, referred to simply as the “active material”). An electrode for a nonaqueous electrolytic secondary cell using such an active material (hereinafter, referred to simply as the “electrode”) is generally formed by applying a slurry containing an electrode active material, a binder and the like to a current collector (hereinafter, an electrode thus obtained will be referred to as the “application type electrode”). However, as the charge/discharge operation is repeated, the active material drastically expands or contracts, and as a result, may be pulverized or divided into tiny particles. When the active material is pulverized or divided into tiny particles, the current collectability of the electrode is decreased and also the contact area of the active material and the electrolytic solution is increased. Such an increase of the contact area promotes the decomposition reaction of the electrolytic solution by the active material, which results in a problem that a sufficient charge/discharge cycle characteristic is not obtained. An application type electrode contains a conductor, a binder and the like therein and so it is difficult to increase the capacitance of the electrode.
Under the circumstances, it has been studied to produce an electrode by forming an active material layer on a current collector using a vacuum process such as a vapor deposition method, a sputtering method, a CVD method or the like, instead of the application type electrode. As compared with the application type electrode, an electrode formed by the vapor deposition method can suppress the active material layer from being divided into tiny particles and also can increase the adhesiveness between the current collector and the active material layer. This improves the electron conductivity in the electrode and also improves the electrode capacitance and the charge/discharge cycle characteristic. Whereas the application type electrode contains a conductor, a binder and the like therein, formation of an active material layer using the vapor deposition method can reduce the amount of, or eliminate, the conductor or the binder present in the electrode. Therefore, the capacitance of the electrode can be essentially increased.
However, even when the vapor deposition method is used, the current collector and the active material layer may be detached from each other, or the current collector may be subjected to a stress to possibly generate wrinkles, due to the expansion and contraction of the active material at the time of charge/discharge. These phenomena reduce the charge/discharge characteristic.
By contrast, Patent Documents 1 and 2 filed by the applicant of the present application propose forming an active material layer by vapor-depositing silicon particles in a direction inclined with respect to the normal to the current collector (oblique vapor deposition). Such an active material layer is formed using the shadowing effect described later, and has a structure in which column-like active material bodies inclined in one direction with respect to the normal to the surface of the current collector are located on the surface of the current collector. According to this structure, a space for alleviating the expansion stress on silicon can be secured between the active material bodies. Therefore, the active material bodies can be suppressed from being detached from the current collector, and the current collector can be suppressed from being wrinkled. As a result, the charge/discharge characteristic can be improved than by the conventional art.
Patent Document 2 proposes forming an active material body grown zigzag by performing a plurality of stages of oblique vapor deposition while switching the vapor deposition direction in order to more efficiently alleviate the expansion stress applied on the current collector. The zigzag active material body is formed as follows, for example.
First, vapor deposition is performed in a first direction inclined with respect to the normal to the current collector to form a first part on the current collector (first stage vapor deposition step). Then, vapor deposition is performed in a second direction inclined oppositely to the first direction with respect to the normal to the current collector to form a second part on the first part (second stage vapor deposition step). Then, vapor deposition is performed further in the first direction to form a third part (third stage vapor deposition step). In this manner, the vapor deposition step is repeated while switching the vapor deposition direction until a desired stacking number is obtained. Thus, an active material body is obtained.
Such an active material body can be formed using, for example, a vapor deposition device described in Patent Document 2. In the vapor deposition device described in Patent Document 2, a fixing table for fixing the current collector is located above an evaporation source. The fixing table is located such that a surface thereof is inclined with respect to a plane parallel to the vapor-depositing surface of the evaporation source (top surface of the vapor-depositing material). Owing to such an arrangement, the vapor-depositing material can be incident on the surface of the current collector in a direction inclined by an arbitrary angle with respect to the normal to the current collector. By switching the inclination direction of the fixing table, the incidence direction of the vapor-depositing material (vapor deposition direction) can be switched. Accordingly, by repeating a plurality of stages of vapor deposition while switching the inclination direction of the fixing table, a zigzag active material body as described above is obtained. It is also described that the incidence direction of the vapor-depositing material is switched by moving the evaporation source or using a plurality of evaporation sources alternately, instead of switching the inclination direction of the fixing table.
However, where the vapor deposition device described in Patent Document 2 is used, vapor deposition is performed on a current collector which is cut in advance into a prescribed size, which decreases the productivity. Accordingly, it is difficult to apply such a vapor deposition device to mass production processes.
Patent Documents 3 through 6 disclose roll-to-roll system vapor deposition devices preferably usable for mass production processes.
Patent Document 3 proposes forming an active material layer by oblique vapor deposition using a roll-to-roll system vapor deposition device. With this vapor deposition device, a sheet-like current collector runs from a supply roll to a take-up roll in a chamber, and a vapor deposition film (active material film) can be continuously formed on the running current collector in a prescribed vapor deposition zone. In this vapor deposition zone, the vapor-depositing material is incident on the surface of the current collector in one direction inclined with respect to the normal to the current collector. Therefore, column-like active material bodies inclined in a particular direction with respect to the normal to the current collector can be formed.
Patent Document 4 discloses various types of roll-to-roll system vapor deposition devices as vapor deposition devices for continuously producing an electrode material for electrolytic capacitors. For example, in one of the disclosed structures, two vapor deposition rolls are provided for one evaporation source, and metal particles evaporated from the evaporation source are vapor-deposited on the surface of the substrate on each vapor deposition roll. Thus, two vapor deposition zones are provided for one evaporation source.
However, it is difficult to continuously form active material bodies grown zigzag as described in Patent Document 2 using the conventional roll-to-roll vapor deposition device described in Patent Document 3 or 4.
As described above, the active material bodies described in Patent Document 2 are formed by performing a plurality of stages of vapor deposition while switching the incidence direction of the vapor-depositing material (vapor deposition direction) to the current collector. With the vapor deposition device described in Patent Document 3, in order to switch the incidence direction of the vapor-depositing material (vapor deposition direction) to the current collector, the location of the vapor deposition zone with respect to the evaporation source needs to be changed. Accordingly, it is difficult to switch the vapor deposition direction in the state where the chamber is kept vacuum. Therefore, a vapor deposition film containing the active material bodies as described above cannot be continuously formed.
The vapor deposition device described in Patent Document 4 is not structured so as to perform oblique vapor deposition from the beginning. It is difficult to control the incidence direction or the vapor deposition direction of the vapor-depositing material with respect to the normal to the current collector. Therefore, it is impossible form active material bodies grown zigzag by controlling the vapor deposition direction thereof.
In addition, according to the conventional vapor deposition devices described above, the vapor deposition zone is formed in only a part of the zone in which the evaporated vapor-depositing material is scattered (vapor deposition possible zone) in the chamber. Therefore, the majority of the vapor-depositing material scattered in the vapor deposition possible zone is not used for vapor deposition. This presents a problem that the utilization factor of the material is very low.
By contrast, Patent Documents 5 and 6 disclose a structure of a roll-to-roll system vapor deposition device having a plurality of vapor deposition zones with different vapor deposition directions. Such a deposition device is provided for the purpose of producing a magnetic tape. Using such a vapor deposition device, a film including layers formed with different vapor deposition directions can be produced.
According to the vapor deposition device shown in FIG. 4 of Patent Document 5, a material substrate formed of a polymer material is transported along three cylindrical rotatable cans controlled to have a temperature of, for example, −10° C. to −15° C. and vapor deposition is performed in two zones (vapor deposition zones) on each rotatable can. In each vapor deposition zone, vapor deposition is performed while a surface of the material substrate opposite to the surface subjected to vapor deposition is cooled by the rotatable can. Therefore, the phenomenon that the material substrate is melted by the heat of the vapor-depositing material can be prevented.
Patent Document 6 discloses a structure of a vapor deposition device including a cooling device for directly cooling a surface of the substrate on which vapor deposition is to be performed. This cooling device is provided for the purpose of preventing the material substrate of a magnetic tape (for example, PET) from being melted.
Hereinafter, the structure of the vapor deposition device disclosed in Patent Document 6 will be described in detail with reference to the figure. FIG. 34 is a cross-sectional view showing a conventional vapor deposition device disclosed in Patent Document 6.
A vapor deposition device 2000 includes rollers 1010 and 1012 for feeding out and taking in a material substrate, a cooling device 1016 and a cooling support 1018 for cooling a material substrate 1014 moving between the rollers 1010 and 1012, an evaporation source 1020 located below a transportation path of the material substrate 1014, and mask shielding plates 1022, 1024 and 1026 for defining a range in which vapor deposition is to be performed on the material substrate 1014. In the vapor deposition device 2000, the material substrate 1014 is fed out from the roller 1010 and cooled by the cooling device 1016. Then, the material substrate 1014 is transported so as to be convexed toward the evaporation source 1020 and is taken up by the roller 1012. The cooling support 1018 is in contact with a rear surface (opposite to a vapor deposition surface) of the material substrate 1014 transported as described above. In such a transportation path of the material substrate 1014, oblique vapor deposition is performed on the material substrate 1014 in a zone 1030 upstream to a part of the material substrate 1014 closest to the evaporation source (the apex of the convexed part) (such a zone will be referred to as the “upstream vapor deposition zone”) and a zone 1032 downstream to the part (such a zone will be referred to as the “downstream vapor deposition zone”). In the upstream vapor deposition zone 1030 and the downstream vapor deposition zone 1032, the vapor-depositing material is incident in directions different from each other with respect to the normal to the material substrate 1014. Therefore, by allowing the material substrate 1014 to pass through these zones, two layers different in the vapor deposition directions can be continuously formed on the material substrate 1014. A bottom end of the vapor deposition zone 1030 (end on the evaporation source side) is defined by the mask shielding plate 1022, and a top end thereof (end on the roller 1010 side) is defined by the mask shielding plate 1024. Similarly, a bottom end of the vapor deposition zone 1032 (end on the evaporation source side) is defined by the mask shielding plate 1022, and a top end thereof (end on the roller 1010 side) is defined by the mask shielding plate 1026.    Patent Document 1: Pamphlet of International Publication WO2007/015419    Patent Document 2: Pamphlet of International Publication WO2007/052803    Patent Document 3: Japanese Laid-Open Patent Publication No. 2007-128659    Patent Document 4: Japanese Patent No. 2704023    Patent Document 5: Japanese Laid-Open Patent Publication No. 53-87706    Patent Document 6: Japanese Laid-Open Patent Publication No. 10-130815